Recently, particulates contained in exhaust gases discharged from internal combustion engines of vehicles, such as buses, trucks and the like, and construction machines and the like have raised serious problems as those particulates are harmful to the environment and the human body.
Conventionally, there have been proposed various ceramic filters which allow exhaust gases to pass through porous ceramics to collect particulates in the exhaust gases, thereby purifying the exhaust gases.
As such a ceramic filter, as shown in FIG. 7, there has been known a honeycomb filter 120, which is prepared as a honeycomb structural body made of silicon carbide and the like, and has a structure in which a plurality of square-pillar shaped porous ceramic members 130 are combined with one another through a sealing material layer 124 that serves as an adhesive to form a ceramic block 125, and a sealing material layer 123 is also formed on the circumference of this ceramic block 125 so as to prevent leakage of exhaust gases.
The honeycomb filter 120 uses the porous ceramic members 130 having a structure as shown in FIG. 8 as constituent components, and a partition wall 133, which are formed therein to separate through holes 131 a large number of which are placed in parallel with one another in the length direction, functions as filters.
In other words, as shown in FIG. 8(b), each of the through holes 131, formed in the porous ceramic member 130, is sealed with a sealing member 132 at one of ends of its exhaust gas inlet side or outlet side, so that exhaust gases that have entered one through hole 131 are discharged from another through hole 131 after having always passed through a partition wall 133 that separates the through holes 131.
Here, as described above, the sealing material layer 123, formed on the periphery, is provided for the purpose of preventing exhaust gases from leaking from the peripheral portion of the ceramic block 125, when the honeycomb filter 120 is installed in an exhaust passage of an internal combustion engine.
Since the honeycomb filter 120 of such a structure has superior heat resistance and provides easy regenerating processes and the like, it has been applied to various large-size vehicles and vehicles with diesel engines. In other words, when the honeycomb filter 120 having such a structure is installed in the exhaust passage of an internal combustion engine, particulates in exhaust gases discharged from the internal combustion engine are captured by the partition wall 133 upon passing through the honeycomb filter 120, so that the exhaust gases are purified.
Moreover, with respect to such a type of honeycomb filter, disclosed is a structure in which the opening area on the exhaust gas inlet side is made larger than the opening area on the exhaust gas outlet side, so that the area of the wall portion through which exhaust gases pass per unit volume is made larger so as to improve the effective volume serving as the filter (for example, see Patent Literatures 1 to 12).
FIG. 9 schematically shows a cross-section perpendicular to the length direction of an exhaust gas filter disclosed in Patent Literature 1 (see FIG. 3 of Patent Literature 1).
In this exhaust gas filter 310, the respective through holes have the same size, and the number of through holes 312, which are sealed at the exhaust gas inlet side, is made smaller than the number of through holes 311, which are sealed at the exhaust gas outlet side. With this arrangement, the opening area on the exhaust gas inlet side is made larger than the opening area on the exhaust gas outlet side, so that the effective volume serving as the filter is improved.
FIG. 10 schematically shows a cross-section perpendicular to the length direction of an exhaust gas filter disclosed in Patent Literature 2 (see Patent Literature 2).
In this exhaust gas filter 320, the opening area and the number of through holes 322, which are sealed at the exhaust gas inlet side, are made different from the opening area and the number of through holes 321, which are sealed at the exhaust gas outlet side. Thus, the opening area on the exhaust gas inlet side is made larger than the opening area on the exhaust gas outlet side, so that the effective volume serving as the filter is improved.
FIG. 11 schematically shows a cross-section perpendicular to the length direction of an exhaust gas filter disclosed in Patent Literature 1 (see FIG. 17 of Patent Literature 1).
In this exhaust gas filter 330, the opening area of through holes 332, which are sealed at the exhaust gas inlet side, is made different from the opening area of through holes 331, which are sealed at the exhaust gas outlet side.
Moreover, in this filter, the number of the through holes 332 and the number of the through holes 331 are the same, and the through holes 331, which are sealed at the exhaust gas outlet side, are mutually made in face-contact with each other through a partition wall. Also in the case of the exhaust gas filter having this structure, the opening area on the exhaust gas inlet side is made larger than the opening area on the exhaust gas outlet side, so that the effective volume serving as the filter is improved.
FIG. 12 schematically shows a cross-section perpendicular to the length direction of an exhaust gas filter disclosed in Patent Literature 3 (see FIG. 5p of Patent Literature 3).
In this exhaust gas filter 340, the opening area of through holes 342, which are sealed at the exhaust gas inlet side, is made different from the opening area of through holes 341, which are sealed at the exhaust gas outlet side.
Moreover, in this filter, the number of the through holes 342 and the number of the through holes 341 are the same, and the through holes 341, which are sealed at the exhaust gas outlet side, are constituted not to have face-contact with each other through a partition wall. Also in the case of the exhaust gas filter having this structure, the opening area on the exhaust gas inlet side is made larger than the opening area on the exhaust gas outlet side, so that the effective volume serving as the filter is improved.
In these conventional filters, the opening area on the gas inlet side is made larger than the opening area on the gas outlet side, so that the effective filtering area of the partition wall is made greater; thus, it becomes possible to collect a large amount of particulates. Moreover, in these filters, an object thereof is to reduce a pressure loss upon collecting the same amount of particulates in comparison with a filter in which, as shown in FIGS. 7 and 8, the cross-sectional shape of all the through holes is a quadrangular shape and the opening area on the gas-inlet side and the opening area on the exhaust gas outlet side are the same.
However, these conventional filters tend to fail to sufficiently achieve the latter object, that is, a reduction in a pressure loss upon collecting the same amount of particulates.
In the above-mentioned filters, it is considered that the following four factors mainly give effects to the pressure loss.
More specifically, those factors are considered to include: (1) an aperture ratio on the exhaust gas inlet side (ΔPa), (2) friction upon passage through through holes (gas inlet side through hole: ΔPb-1, gas outlet side through hole: ΔPb-2), (3) resistance upon passage through a partition wall (ΔPC), and (4) resistance exerted upon passage through collected particulates (ΔPd). Here, among these, the effect exerted by (4) resistance exerted upon passage through collected particulates (ΔPd) is considered to be greatest.
Here, in the case of the filters having the structures shown in FIGS. 9 to 12, the initial pressure loss (pressure loss in a state without particulates collected) tends to become higher in comparison with the filter in which, as shown in FIGS. 7 and 8, the cross-sectional shape of all the through holes is a quadrangular shape and the opening area on the exhaust gas inlet side and the opening area on the exhaust gas outlet side are the same. The reason for this is because, although the pressure loss caused by ΔPa and ΔPb-1 is slightly reduced, the pressure loss caused by ΔPb-2 and ΔPc becomes higher.
Moreover, with respect to the pressure loss after collection of particulates in a filter having each of structures as shown in FIGS. 9 to 12, the filters having the structures shown in FIGS. 9 to 11 have a partition wall commonly possessed by the gas flow-in through holes. In the filters having this structure, as shown in FIG. 13, exhaust gases first flow from the gas flow-in through hole 1311 side to the gas flow-out through hole 1312 side through flow passages “a” via the partition wall commonly possessed by the gas flow-in through hole 1311 and the gas flow-out through hole 1312. At this time, particulates are captured by the partition wall commonly possessed by the gas flow-in through hole 1311 and the gas flow-out through hole 1312 (see FIG. 13(a)).
Thereafter, as the particulates 1313 are collected on the partition wall commonly possessed by the gas flow-in through hole 1311 and the gas flow-out through hole 1312, so that the pressure loss in the partition wall becomes higher due to ΔPd, and exhaust gases are allowed to flow from the gas flow-in through hole 1311 side to the gas flow-out through hole 1312 side through flow passages “b” via a partition wall commonly possessed by the gas flow-in through holes 1311 (see FIG. 13(b)).
In this case, of the partition wall commonly possessed by the gas flow-in through holes 1311, it is considered that: the exhaust gases start to flow at the portion closest to the partition wall commonly possessed by the gas flow-in through hole 1311 and the gas flow-out through hole 1312; and the gas flow-in portion gradually expands to finally allow the entire partition wall forming the gas flow-in through hole 1311 to serve as an effective filtering region.
FIGS. 13(a) and 13(b) are schematic diagrams for describing flow passages of exhaust gases in the conventional filters.
In a honeycomb structural body of this type, when the amount of particulates accumulated on the partition wall commonly possessed by the gas flow-in through hole 1311 and the gas f low-out through hole 1312 is large, it has been difficult to reduce the pressure loss upon collection of particulates.
Moreover, Patent Literatures 3 and 4 disclose a filter in which an average porosity is more than 10% or less and pores have an average pore diameter of 2 to 15 μm, with individual pore diameters distributed in the almost entire range from 0.5 to 70 μm.
The present inventors have also studied methods for increasing the pore diameter in order to reduce the pressure loss. However, as a result of the studies, it has been found that unexpectedly, even when the pore diameter is made larger, the pressure loss is not lowered.    Patent Literature 1: JP-B 03-49608 (1991) (FIGS. 3, 17 and the like), U.S. Pat. No. 4,417,908, JP-A 58-196820 (1983)    Patent Literature 2: JP-U 58-92409 (1983)    Patent Literature 3: U.S. Pat. No. 4,364,761 (FIG. 5p and the like), JP-A 56-124417 (1981), JP-A 62-96717 (1987)    Patent Literature 4: U.S. Pat. No. 4,276,071    Patent Literature 5: U.S. Pat. No. 4,420,316    Patent Literature 6: U.S. Pat. No. 4,420,316    Patent Literature 7: JP-A 58-150015 (1983)    Patent Literature 8: JP-A05-68828 (1993), Japanese Patent No. 3130587    Patent Literature 9: FR2789327    Patent Literature 10: WO02/100514    Patent Literature 11: WO02/10562, DE10037403    Patent Literature 12: WO03/20407, US Patent 2003-41730, U.S. Pat. No. 6,696,132